Biaxially oriented multi-layer polypropylene (BOPP) films are known to comprise a homopolypropylene layer which is sandwiched between two thin skin layers. Said skin layers provide surface properties to the BOPP film. They could correspond to the same grade as the core layer but with an appropriate formulation (e.g. with anti-block additives). They could also correspond to a polypropylene comprising at least one comonomer when sealing properties are required. In the latter case, said skin layers are also called heat seal layers.
Continuous efforts are made in order to have improved BOPP heat seal layers. The optimal desired product will ideally have a combination of characteristics consisting in having low seal initiation temperature that do not lead to sticking on the rolls during BOPP production (in a tenter process); having good hot tack properties to allow the use of the BOPP film having the heat seal layer in a vertical-form-fill-seal packaging system in the broadest possible range of temperature and speed; having low xylene solubles and/or hexane solubles content to use the BOPP film with the heat seal layer in food packaging according to the legislation in force for food packaging; having good optical properties such as high transparency, low haze and high gloss; improved resistance to scratch, having good slip properties, good anti-block properties, good anti-static properties, good printability properties and good aptitude to metallization.
EP 1 588 843 discloses a polyolefin based biaxially oriented multi-layer film having at least one surface layer comprising a propylene-ethylene random copolymer which is obtained through sequential polymerization catalyzed by a metallocene component. In such sequential polymerization, the propylene-ethylene random copolymer component produced in the first step has an ethylene content of 1 to 7 wt % and the propylene-ethylene random copolymer component produced in the second step has an ethylene content of 6 to 15 wt % higher than that of the polymer component obtained in the first step. The propylene-ethylene random copolymer has a high melt flow rate of at least 16 g/10 min, which increases the risk of heat seal layer thickness issues when used in a multi-layer BOPP production. In addition, the resin used for the production of said films comprises high xylene solubles content.
EP 1 270 651 discloses a polymer film comprising a propylene random copolymer with the comonomer being ethylene or an alpha-olefin having at least four carbon atoms and a total comonomer content of 4.5 to 12 mol % wherein the sealing initiation temperature SIT of the film is the melting temperature (Tm)—30 DEG C. or less, preferably (Tm)—33 DEG C. or less and the xylene solubles is from 4 to 15 wt %, preferably from 6 to 10 wt %. The melting temperature Trn of the copolymer is 135 DEG C. or higher.
For most applications of BOPP heat seal layers, high hexane solubles content is not desired. For instance, in food packaging, the hexane solubles content should be low in order to be in conformity with the legislation.
It is an object of the present invention to provide a propylene-comonomer block copolymer for use as heat seal layer thus that the heat seal layer have low seal initiation temperature.
It is another object of the present invention to provide a propylene-comonomer block copolymer having low hexane solubles content.
It is another object of the present invention to provide a propylene-comonomer block copolymer for use as heat seal layer designed to provide a good compromise between low hexane solubles content and low seal initiation temperature.
It is another object of the present invention to provide a propylene-comonomer block copolymer for use as heat seal layer, designed to provide a good compromise between low hexane solubles content, low seal initiation temperature and good results in stickiness.
It is another object of the present invention to provide a heat seal layer having improved optical properties.
It is another object of the present invention to provide a heat seal layer having improved hot tack properties and surface treatment retention.